Share the library.
Keep each book unique.

Mini Library Manager now separates information shared by every Book from information owned by one book.

Learning goalDistinguish class attributes from instance attributes, update shared state, and recognize when one instance shadows a class attribute.

01 Learn02 Try03 Predict

01 Learn

One library, many individual books.

Put a class attribute directly inside Book to share it. Put an instance attribute on self in __init__ to give each book its own value.

class Book:
    library_name = "Mini Library"

    def __init__(self, title, author):
        self.title = title
        self.author = author
        self.is_available = True

book = Book("Dune", "Frank Herbert")
print(Book.library_name)
print(book.title)

library_name belongs to the Book class and is shared. title belongs to book, because self.title creates an attribute on that one instance.

Mini Library
Dune

Changing Book.library_name updates the shared lookup for instances that do not have their own library_name. Assigning book.library_name creates a separate instance attribute that shadows the class attribute.

↳

Attribute lookup order: When Python reads book.library_name, it checks the book first and then the Book class. An instance value therefore hides the shared value for that one book.

Shared value and one shadowed value

class Book:
    library_name = "Mini Library"

    def __init__(self, title, author):
        self.title = title
        self.author = author
        self.is_available = True

book_one = Book("Dune", "Frank Herbert")
book_two = Book("The Hobbit", "J. R. R. Tolkien")

Book.library_name = "City Library"
book_one.library_name = "Personal Shelf"

print(book_one.library_name)
print(book_two.library_name)

Book.library_name changes the shared name to City Library. book_one then gets its own Personal Shelf value, while book_two still uses the class value.

Where attributes belong

class Book:
    library_name = "Mini Library"  # class attribute

    def __init__(self, title):
        self.title = title            # instance attribute

Use a class attribute for one value shared by all books. Use an instance attribute for data that can differ from book to book.

Trace an attribute lookup

1. Read the instance firstbook_one.library_name first looks for library_name on book_one.
2. Fall back to the classWithout an instance value, Python reads Book.library_name.
3. Change shared stateBook.library_name = "City Library" changes the value used by unshadowed instances.
4. Shadow one instancebook_one.library_name = "Personal Shelf" affects only book_one.

Common mistakes

Using a shared list for per-book dataA mutable class attribute such as tags = [] is shared by every instance. Create a new list on self in __init__ when each book needs its own tags.
Expecting an instance assignment to update all booksbook_one.library_name creates or changes book_one’s value only; it does not change Book.library_name.
Calling every attribute sharedtitle, author, and is_available are instance attributes because they are assigned through self.
Forgetting the class name for a shared updateUse Book.library_name to deliberately update the value shared by the class.

02 Try

Python Playground

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Change the shared library name, then give one book its own name and compare both books.

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03 Predict

What will this program print?

First follow the class update, then notice that book_one receives its own attribute.

class Book:
    library_name = "Mini Library"

    def __init__(self, title, author):
        self.title = title
        self.author = author
        self.is_available = True

book_one = Book("Dune", "Frank Herbert")
book_two = Book("The Hobbit", "J. R. R. Tolkien")

Book.library_name = "City Library"
book_one.library_name = "Personal Shelf"

print(book_one.library_name)
print(book_two.library_name)

Practice independently

Mini Challenge — Label the library

Use one class attribute for the library name and instance attributes for a new book’s data.

Your goalDefine Book.library_name, create two books with their own titles, change the shared name, and give one book a personal label.

library_labels_challenge.py

Optional hints

Write library_name = "Mini Library" directly in the class, outside __init__.
Use Book.library_name for the shared update and book_one.library_name for the one-book label.

Ask Sloby for a hint